The separation of convolutive mixtures aims to estimate the individual sound signals in the presence of other such signals in reverberant environments. As sound mixtures are almost always convolutive in enclosures, their separation is a useful pre-processing stage for speech recognition and speaker identification problems. Other direct application areas also exist such as in hearing aids, teleconferencing, multichannel audio and acoustical surveillance.
Our earlier patent application published as WO 2009/050487 discloses a system for separating a mixture of acoustic signals from a plurality of sources which comprises a sensor array comprising a plurality of pressure sensors and a processor arranged to receive signals from the sensors, and derive from them a series of sample values of directional pressure gradient, identify a plurality of frequency components of the signals, and define an associated direction for each frequency component. The system is then arranged to identify a subset of the frequency components with a source, thereby to define an acoustic signal for that source. Signals for several sources can be defined. In order to provide three dimensional source separation, a three dimensional array of sensors can be used, for example a tetrahedral array.
Tetrahedral ambisonic surround sound microphones (herein referred to as “tetramics”) conventionally comprise four capsule microphones arranged according to a regular tetrahedron. These are well known, and the natural choice for a system such as that of WO 2009/050487. Typically each microphone is a cardioid or sub-cardioid, whose diaphragm is centred at the corner of a regular tetrahedron, and whose direction as defined by the axis of the diaphragm points outwards from the central of the tetrahedron. Typically the four microphones are held in place using customised mechanical assemblages and fixings.
Whilst well suited to high-fidelity ambisonic applications, tetramics are not well suited to many consumer applications. First, they cannot easily be manufactured using the production techniques employed by the consumer electronics industry such as rigid or semi-rigid printed circuit boards. Second, they require manual assembly and hence are more expensive to manufacture compared to an equivalent device produced using automated processes. Third, typically the volume of the tetramic is large compared to the volume of the four component microphones.
These and other matters have limited the application of tetramics to high-fidelity professional audio recording applications. Specifically, tetramics are not generally amenable to being embedded into compact consumer devices such as mobile phones, laptop computers, hearing aids, or into low-cost consumer devices such as lanyard microphones.